FIG. 10 is a view showing the connection form of a charger and a power supply. Referring to FIG. 10, a charger 201 has a DC input jack 202 to receive supply of a DC power. The charger 201 can charge batteries 203 and 204. A car battery cable 205 has, at its two ends, a plug 206 to be connected to the receptacle of the cigarette lighter of a car and a DC input plug 207 which supplies the DC 12- or 24-V power of the car battery to the DC input jack 202. A power adapter 208 converts an AC power supplied from an AC input plug 209 into a DC 9.5-V power and supplies the DC power to the DC input jack 202 through a DC input plug 210.
As shown in FIG. 10, the charger 201 capable of receiving power from a plurality of power sources has a dedicated 3-terminal power supply connector (DC input jack 202) and generates the charging voltage of the battery 203 or 204 through a DC/DC converter.
The charger of this type always generates the charging voltage by using the DC/DC converter. For this reason, when a DC power equal to the charging voltage of the battery is input from an external power supply apparatus, the charging voltage of the battery cannot be obtained. Hence, the battery cannot fully be charged.
Charging is sometimes executed by using a power supply apparatus which can supply a DC power to an electronic device and simultaneously execute constant-voltage/constant-current control according to the battery charging conditions. Generally, the power supply apparatus is designed to cope with a rush current of an electronic device by canceling constant-current control or changing the constant-current control value when the output voltage drops. Hence, under normal quick charging start conditions, an excessively large charging current may flow to the battery to damage it or shorten its service life.
A charger which executes battery charging control by using a power supply for executing constant-voltage/constant-current control for battery charging or a power adapter for receiving an AC power and, for example, a charger shown in FIG. 15 is known. Referring to FIG. 15, the charger 201 executes constant-voltage/constant-current control of a DC power supplied from the plug 206 to charge the battery 203. Generally, backflow of the DC power from the battery 203 is prevented by arranging a blocking element such as a diode on the output side of the power adapter or on the input side of the charger 201.
However, when a blocking element is inserted to the power supply line, as described above, a large-capacitance element that allows the charging current must be used. This is disadvantageous for cost. In some cases, no sufficient charging voltage can be supplied to the battery 203 due to a voltage drop in the blocking element, or the battery cannot fully be charged due to a variation between elements.